TTL indexes for auto-expiry in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What happens when a TTL index expires a document?

Consider a MongoDB collection with a TTL index set to expire documents 3600 seconds after the createdAt field. What is the expected output after the TTL monitor runs?

MongoDB

db.collection.find({})

A Documents older than 3600 seconds based on <code>createdAt</code> are automatically deleted.

B Documents are archived to another collection after 3600 seconds.

C Documents are updated with a new timestamp after 3600 seconds.

D Documents remain unchanged; TTL indexes only create an index but do not delete.

💡 Hint

Think about what TTL indexes are designed to do automatically in MongoDB.

TTL indexes automatically remove documents after the specified time has passed since the indexed date field. This helps in auto-expiring data without manual deletion.

📝 Syntax

intermediate

Which command correctly creates a TTL index to expire documents after 24 hours?

Choose the correct MongoDB shell command to create a TTL index on the timestamp field that expires documents after 24 hours.

A db.collection.createTTLIndex({ timestamp: 1 }, { expireAfterSeconds: 86400 })

B db.collection.createIndex({ timestamp: 1 }, { ttl: 86400 })

C db.collection.createIndex({ timestamp: 1 }, { expireAfterSeconds: 86400 })

D db.collection.createIndex({ timestamp: 1 }, { expireAfter: 86400 })

💡 Hint

Look for the correct option name for the expiration time in the index options.

The correct option to specify expiration time in seconds is expireAfterSeconds. The method is createIndex.

❓ optimization

advanced

How to optimize TTL index performance for high write throughput?

You have a collection with a TTL index on createdAt and very high write volume. Which approach optimizes TTL index performance?

A Create multiple TTL indexes on different date fields to distribute load.

B Use a single TTL index on <code>createdAt</code> and shard the collection by <code>createdAt</code>.

C Avoid TTL indexes and manually delete expired documents with a background job.

D Use a compound index with <code>createdAt</code> and a frequently updated field.

💡 Hint

Consider how sharding and indexing affect write performance and TTL expiration.

Sharding by the TTL indexed field helps distribute writes and TTL deletions evenly, improving performance. Multiple TTL indexes or compound indexes can cause overhead.

🔧 Debug

advanced

Why does the TTL index not delete expired documents?

A TTL index was created on expireAt with expireAfterSeconds: 0, but expired documents remain. What is the likely cause?

A The <code>expireAt</code> field is not a date type or missing in documents.

B The TTL monitor runs only once a day, so documents are not deleted yet.

C The <code>expireAfterSeconds</code> value must be greater than zero.

D TTL indexes only work on the <code>createdAt</code> field by default.

💡 Hint

Check the data type and presence of the indexed field in documents.

TTL indexes require the indexed field to be a valid date type and present in documents. If missing or wrong type, TTL deletions won't occur.

🧠 Conceptual

expert

What is the behavior of TTL indexes with clustered collections?

In MongoDB, when using clustered collections, how does TTL index behavior differ compared to regular collections?

A TTL indexes can be created on any field, but expiration is delayed due to clustering.

B TTL indexes are not supported on clustered collections; expiration must be manual.

C TTL indexes automatically expire documents, but only if the clustered key is a date field.

D TTL indexes work the same, but the clustered key must be the TTL indexed field.

💡 Hint

Think about how clustered collections organize data and index requirements.

TTL indexes on clustered collections require the TTL field to be the clustered key because the data is organized by that key. This ensures efficient expiration.